A multicarrier transmission system, as opposed to a single carrier system, uses a set of different frequencies distributed in the transmission channel frequency band to carry the data. The main advantage is that the bit rate can be adjusted for each carrier, according to the noise and distortion power in the vicinity of this carrier. Thus, a better approximation of the theoretical information capacity limit can be expected and, particularly, poor quality channels can be exploited, like some wireless communication channels, power lines or the telephone subscriber lines at high or very high frequencies. A detailed description of existing multicarrier transmission systems and their merits compared to single carrier systems is given in the book by W. Y. Chen: <<DSL-Simulation Techniques and Standards Development for Digital Subscriber Line Systems>>, MacMillan Technical Publishing, Indianapolis, USA, 1998.
In order to efficiently perform multicarrier transmission, two basic approaches have been considered so far. The first one and most widely used is called OFDM (Orthogonal Frequency Division Multiplexing) or DMT (Digital Multi-Tone) and it is based on the FFT (Fast Fourier Transform). It has been a subject of intense research and development efforts. In that scheme, the data are arranged in blocks which are transmitted by orthogonal carriers, and separated by guard times, which have to be greater than the channel impulse response, to preserve the orthogonality of the carriers at the receiving side. In spite of the potential of the approach, OFDM/DMT suffers from a number of weaknesses, which, overall, make it perform hardly better than single carrier transmission: a complex time equalizer has to be introduced in front of the receiver to reduce the channel impulse response length, very precise time synchronization is necessary, a long initialization phase is required, and the carriers, and subchannels, are poorly separated, which reduces the capacity of the system in the presence of jammers. In fact, a good quality channel is necessary for that scheme to work properly. Ample documentation can be found in the literature and a good list of references is given in the book by W. Y. Chen.
A second approach aims at overcoming some of the OFDM/DMT limitations through the use of more sophisticated transforms than the FFT, namely, lapped transforms and wavelet transforms. The idea is to improve the separation between carriers, or subchannels. A lot of theoretical work has been done about this subject, see for example the paper by S. D. Sandberg and M. A. Tzannes: <<Overlapped Discrete Multitone Modulation for High Speed Copper Wire Communications>>, IEEE-JSAC, Vol. 13, No9, December 1995. Although they improve the subchannel separation, the lapped and wavelet approaches still retain some of the crucial OFDM/DMT limitations and particularly the time synchronization requirements.
In fact, the ideal approach for multicarrier transmission is one in which the subchannels are made independent and this is achieved by filter banks. This has been recognized a long time, and an efficient implementation of filter banks for transmission systems, based on the combination of an FFT processor with a polyphase network, has been presented in the paper by M. Bellanger and J. Daguet: <<TDM-FDM Transmultiplexer: Digital Polyphase and FFT>>, IEEE Transactions on Communications, Vol. COM-22, September 1974. Later on, a technique called OQAM (Orthogonal Quadrature Amplitude Modulation) has been proposed for multicarrier transmission with filter banks, see the paper by B. Hirosaki, <<An Orthogonally Multiplexed QAM System Using the Discrete Fourier Transform>>, IEEE Trans. on Communications, Vol. COM-29, July 1981. Its main feature is that the subchannel sampling rate is twice the Nyquist subchannel frequency, or subchannel spacing, and the data are transmitted alternatively on the real and the imaginary part of the complex signal in any subchannel, with, again, an alternation between two adjacent subchannels. With this technique, intersymbol interference is eliminated in a subchannel and between adjacent subchannels. The distortions introduced by the transmission channel can be eliminated by a multibranch equalizer in each subchannel. Recently, it has been shown that a single branch equalizer can be used, see the paper by L. Qin and M. Bellanger, <<Equalization issues in Multicarrier Transmission Using Filter Banks>>, Annals of Telecommunications, Vol. 52, No1–2, January 1997.
In spite of its potential theoretical advantages, the OQAM multicarrier approach is seldom considered for implementation in practical systems. A key reason is that the real/imaginary alternation principle raises problems, which have not been adequately solved thus far, for the equalization algorithms, the carrier synchronization and the system timing organization.